Chemically synthesized and assembled electronic devices

ABSTRACT

A route to the fabrication of electronic devices is provided, in which the devices consist of two crossed wires sandwiching an electrically addressable molecular species. The approach is extremely simple and inexpensive to implement, and scales from wire dimensions of several micrometers down to nanometer-scale dimensions. The device of the present invention can be used to produce crossbar switch arrays, logic devices, memory devices, and communication and signal routing devices. The present invention enables construction of molecular electronic devices on a length scale than can range from micrometers to nanometers via a straightforward and inexpensive chemical assembly procedure. The device is either partially or completely chemically assembled, and the key to the scaling is that the location of the devices on the substrate are defined once the devices have been assembled, not prior to assembly.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a divisional of Application Ser. No. 09/282,048 filed Mar. 29,1999, now U.S. Pat. No. 6,459,095.

The present application is related to the followingapplications/patents: Ser. Nos. 09/280,225, now U.S. Pat. No. 6,314,019,issued November 6, 2001 (“Molecular Wire Crossbar Interconnects forSignal Routing and Communications”); 09/280,189, now U.S. Pat. No.6,128,214, issued Oct. 3, 2000 (“Molecular Wire Crossbar Memory”);09/282,045 (“Molecular Wire Crossbar Logic”); 09/282,049, now U.S. Pat.No. 6,256,767, issued Jul. 3, 2001 (“Demultiplexer for a Molecular WireCrossbar Network (MWCN Demux)”); and 09/280,188 (“Molecular WireTransistors”), all filed on even date herewith. The present applicationis the foundational application, upon which the related applicationsdepend for construction of the various devices and apparati disclosedand claimed therein.

This invention was made with Government support under (DMR-9726597)awarded by the National Science Foundation. The Government has certainrights in this invention.

TECHNICAL FIELD

The present invention relates generally to electronic devices whosefunctional length scales are measured in nanometers, and, moreparticularly, to simple devices used as building blocks to form morecomplicated structures, and to the methods for forming such devices.Devices both of micrometer and nanometer scale may be constructed inaccordance with the teachings herein.

BACKGROUND ART

The silicon (Si) integrated circuit (IC) has dominated electronics andhas helped it grow to become one of the world's largest and mostcritical industries over the past thirty-five years. However, because ofa combination of physical and economic reasons, the miniaturization thathas accompanied the growth of Si ICs is reaching its limit. The presentscale of devices is on the order of tenths of micrometers. New solutionsare being proposed to take electronics to ever smaller levels; suchcurrent solutions are directed to constructing nanometer-scale devices.

Prior proposed solutions to the problem of constructing nanometer-scaledevices have involved (1) the utilization of extremely fine scalelithography using X-rays, electrons, ions, scanning probes, or stampingto define the device components; (2) direct writing of the devicecomponents by electrons, ions, or scanning probes; or (3) the directchemical synthesis and linking of components with covalent bonds. Themajor problem with (1) is that the wafer on which the devices are builtmust be aligned to within a small fraction of the size of the devicefeatures in at least two dimensions for several successive stages oflithography, followed by etching or deposition to build the devices.This level of control does not scale well as device sizes are reduced tonanometer scale dimensions. It becomes extremely expensive to implementas devices are scaled down to nanometer scale dimensions. The majorproblem with (2) is that it is a serial process, and direct writing awafer full of complex devices, each containing trillions of components,could well require many years. Finally, the problem with (3) is thathigh information content molecules are typically macromolecularstructures such as proteins or DNA, and both have extremely complex and,to date, unpredictable secondary and tertiary structures that cause themto twist into helices, fold into sheets, and form other complex 3Dstructures that will have a significant and usually deleterious effecton their desired electrical properties as well as make interfacing themto the outside world impossible.

There remains a need for a basic approach to form nanometer-scaledevices that can be used to form more complex circuits and systems, andthat scale readily and cheaply down to nanometer-scale dimensions.

DISCLOSURE OF INVENTION

In accordance with the present invention, a route to the fabrication ofelectronic devices is provided, in which the devices consist of twocrossed wires sandwiching an electrically addressable molecular species.The approach is extremely simple and inexpensive to implement, andscales from wire dimensions of several micrometers down tonanometer-scale dimensions. The device of the present invention can beused to produce crossbar switch arrays, logic devices, memory devices,and communication and signal routing devices.

The present invention enables construction of molecular electronicdevices on a length scale than can range from micrometers to nanometersvia a straightforward and inexpensive chemical assembly procedure. Thedevice is either partially or completely chemically assembled, and thekey to the scaling is that the location of the devices on the substrateare defined once the devices have been assembled, not prior to assembly.

The electronic device of the present invention, in one realization, is aquantum-state molecular switch comprising an electrically adjustabletunnel junction between two wires. Only at the intersection of the twowires is an actual device defined. The exact position of thisintersection is not important for this architecture. The moleculardevices sandwiched between the wires can be electrochemically oxidizedor reduced. Oxidation or reduction of the molecule forms the basis of aswitch. Oxidation or reduction will affect the tunneling distance or thetunneling barrier height between the two wires, thereby exponentiallyaltering the rate of charge transport across the wire junction. Sometypes of molecules can be cycled reversibly, while others will actirreversibly. The chemical state of the molecular switches determinesthe tunneling resistance between the two wires.

The present invention solves several problems that currently plaguecurrent solid state electronic device technology. First, the fundamentaldevice unit is a molecule or a layer of molecules at the junction of twowires, and so the devices will scale down from wires of micrometerlength scales to wires of molecular length scales (a nanometer, forexample) without appreciable change in device operation. Second,molecular devices are voltage, not electric field, addressable. Thismeans that molecular switches can be set at one voltage, and the stateof the switch can be read at another voltage (either smaller inmagnitude or a different polarity), and only two wires are required forthe entire process. In most solid-state devices, a total of four wiresare required to set and subsequently read the state of a switch. Theseinclude two wires that are required to set the state of a switch, andtwo different wires that are required to read the state of that switch.Third, the devices that are fabricated are extremely versatile, and canbe configured to carry out any number of tasks, ranging from memory tologic to communication and signal routing to energy storage. Finally,since only two wires are needed to address and read these devices, andsince the device itself is defined not by high resolution lithographictemplating, but rather by the relatively arbitrary intersection of twowires, the fabrication process for these wires is substantially simplerand more tolerant of manufacturing deficiencies than is the current art.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic representation of two crossed wires, with atleast one molecule at the intersection of the two wires, in accordancewith the invention;

FIG. 1B is a perspective elevational view, depicting the device shown inFIG. 1A;

FIG. 2 is a schematic representation of the steps that might be employedto reduce this invention to practice using lithographically deposited(micrometer or sub-micrometer scale diameter) wires;

FIG. 3 is a schematic representation of the steps that might be employedto reduce this invention to practice using chemically fabricated(nanometer-scale diameter) wires;

FIGS. 4A, 4B, and 4C depict the molecular structure of three molecularcompounds, each of which was utilized to successfully reduce thisinvention to practice;

FIG. 5 depicts the energy level diagram of devices made according to thescheme outlined in FIG. 2, and utilizing the molecule depicted in FIG.4C;

FIG. 6A, on coordinates of current (in amps) and voltage (in volts), isa plot depicting the electrical responses that were measured when adevice of the present invention was reduced to practice using the schemeoutlined in FIG. 2, and utilizing the specific molecule depicted in FIG.4C;

FIG. 6B, on coordinates of normalized density of states (dI/dV×V/I) andvoltage (in volts), is a plot depicting an experimental measurement ofthe electronic energy levels of the molecular switch that was measuredin FIG. 6A; the measurements indicate that the wire/molecule/wirejunction acts as a resonant tunneling resistor in the ‘switch-closed’state, and as a tunneling resistor in the ‘switch-open’ state; and

FIG. 7 is a schematic representation of a two-dimensional array ofswitches of the present invention, depicting a 6×6 crossbar switch.

BEST MODES FOR CARRYING OUT THE INVENTION

Definitions

As used herein, the term “self-aligned” as applied to “junction” meansthat the junction that forms the switch and/or other electricalconnection between two wires is created wherever two wires, either ofwhich may be coated or functionalized, cross each other, because it isthe act of crossing that creates the junction.

The term “self-assembled” as used herein refers to a system thatnaturally adopts some geometric pattern because of the identity of thecomponents of the system; the system achieves at least a local minimumin its energy by adopting this configuration.

The term “singly configurable” means that a switch can change its stateonly once via an irreversible process such as an oxidation or reductionreaction; such a switch can be the basis of a programmable read-onlymemory (PROM), for example.

The term “reconfigurable” means that a switch can change its statemultiple times via a reversible process such as an oxidation orreduction; in other words, the switch can be opened and closed multipletimes, such as the memory bits in a random access memory (RAM).

The term “bi-stable” as applied to a molecule means a molecule havingtwo a relatively low energy states. The molecule may be eitherirreversibly switched from one state to the other (singly configurable)or reversibly switched from one state to the other (reconfigurable).

Micron-scale dimensions refers to dimensions that range from 1micrometer to a few micrometers in size.

Sub-micron scale dimensions refers to dimensions that range from 1micrometer down to 0.04 micrometers.

Nanometer scale dimensions refers to dimensions that range from 0.1nanometers to 50 nanometers (0.05 micrometers).

Micron-scale and submicron-scale wires refers to rod or ribbon-shapedconductors or semiconductors with widths or diameters having thedimensions of 1 to 10 micrometers, heights that can range from a fewtens of nanometers to a micrometer, and lengths of several micrometersand longer.

Crossed Wire Switch

The essential device features are shown in FIGS. 1A-1B. A crossed wireswitch 10 comprises two wires 12, 14, each either a metal orsemiconductor wire, that are crossed at some non-zero angle. In betweenthose wires is a layer of molecules or molecular compounds 16, denoted Rin FIGS. 1A and 1B. The particular molecules 18 (denoted R_(s)) that aresandwiched at the intersection of the two wires 12, 14 are identified asswitch molecules. When an appropriate voltage is applied across thewires, the switch molecules are either oxidized or reduced. When amolecule is oxidized (reduced), then a second species is reduced(oxidized) so that charge is balanced. These two species are then calleda redox pair. One example of this device would be for one molecule to bereduced, and then a second molecule (the other half of the redox pair)is oxidized. In another example, a molecule is reduced, and one of thewires is oxidized. In a third example, a molecule is oxidized, and oneof the wires is reduced. In a fourth example, one wire is oxidized, andan oxide associated with the other wire is reduced. In all cases,oxidation or reduction will affect the tunneling distance or thetunneling barrier height between the two wires, thereby exponentiallyaltering the rate of charge transport across the wire junction, andserving as the basis for a switch.

The electrical tasks performed by these devices are largely determinedby the types of wires (electrodes) and the interwire materials that areused. Table I presents the various types of devices that might befabricated from various combinations of the wires 12, 14 in FIGS. 1A-1B.

TABLE I Wire (Electrode) Materials Semi- conductor- Semi- Metal- Metal-Metal- conductor Semiconductor- Device metal metal semicon- (p-nsemiconductor Type (same) (different) ductor junction) (heterojunctionResistor X X X Tunneling X X X resistor Resonant X X X tunnelingresistor Diode X X X X Tunneling X X X X diode Resonant X X X Xtunneling diode Battery X X X

Depending on the molecules or materials that are used between the wires(the electrodes), each junction can either display the types ofelectrical function described below immediately on contact of the wiresor the junction can have a switching function that acts to connect ordisconnect the two wires together electrically. This switch can eitherbe singly configurable or reconfigurable. In the first case, the initialstate of the switch is open or closed. Electrically biasing the switchbeyond a particular threshold voltage that is determined by thematerials in the junction, which is essentially an electrochemical cell,oxidizes or reduces the material or molecules between the wires toirreversibly close or open the switch, respectively, thus permanentlyreversing its initial state. In the second case, by cycling the polarityand magnitude of the voltage on the switch beyond the appropriatethreshold values, it is possible to reversibly oxidize or reduce theproperly selected materials or molecules to close or open the switchmany times. In either case, when closed, the type of electricalconnection that is made between the wires depends upon the materialsfrom which the wires (or electrodes) are fabricated as well as theidentity of the molecules or materials between the wires.

Table I above shows a matrix of the various types of functions that canbe obtained from various combinations of electrode materials andmaterials or molecules used in the junction. A resistor has a linearcurrent-voltage characteristic, and is made by intentionallyover-reducing the junction between various types of wires to essentiallyform a short circuit between the wires. The opposite of this process isto over-oxidize a junction, which will consume the wire in a localizedregion and effectively break the wire (create an open circuit) in thatwire at the position of the junction. A tunneling resistor maintains athin, approximately 2 nanometer thick, insulating barrier between wiresand has an exponential current-voltage characteristic. In the case thatjunction molecules or materials have a sharply defined energy stateinside the band gap of an electrically insulating barrier that can beaccessed by electrically biasing the junction, the connection betweenthe wires can exhibit a flow of electrical current that is dominated bythe process of resonant tunneling. The resonant tunneling can produceone or more inflection points in the otherwise exponentialcurrent-voltage characteristic of a tunneling resistor. A diode is ajunction that passes current more easily in one direction than in theother, and thus has an asymmetry in the current-voltage characteristicfor positive and negative voltages. A tunneling diode has both thepositive-negative voltage asymmetry of the diode and the exponentialcurrent-voltage characteristic of the tunneling resistor. A resonanttunneling diode has a positive-negative voltage asymmetry plus it has apeak in its current-voltage characteristic, such that over a restrictedrange of increasing magnitude of the voltage the magnitude of thecurrent actually decreases, a phenomenon that is known as negativedifferential resistivity. Finally, a battery is a circuit element thatacts to hold a constant voltage difference between its electrodes aslong as the battery is sufficiently charged, e.g., there is a sufficientsupply of oxidizing and reducing agents separated by an insulatingbarrier. Charging the battery is accomplished by placing the appropriatevoltage across the junction, which as stated before is anelectrochemical cell, to only partially oxidize or reduce the materialor molecules in the junction. In general, any real junction betweenwires formed by the processes described above will actually have two ormore of the electrical functions described, with the effective circuitelements connected in series.

Thus, the present invention may be executed with any number of metallicor semiconducting wire/molecule combinations, depending on the deviceproperties desired from the assembled circuit.

Fabrication of Wire Electrodes

1. Process-Defined Wires (defined as wires that are prepared byconventional electronic-circuit processing techniques; wires aretypically prepared on a substrate as part of a circuit):

Metallic and semiconductor wires, with diameters ranging from severalmicrometers to a single micrometer (defined as micrometer-scale), orwith diameters ranging from a single micrometer down to 40 nanometers(defined as sub-micrometer scale) in diameter, may be prepared usingwell-established art, including lithographic (optical, ultraviolet, orelectron beam) technologies. These wires normally have a ribbon shape orrectangular cross section, although circular cross sections are notprecluded, with the width of the wire being determined by thelithographic process used to define the wire and its height beingdefined by the amount of material deposited in the region defined bylithography.

2. Chemically-Prepared Wires (these wires are prepared by techniquesother than conventional electronic processing technology; wires aretypically prepared as a bulk material, rather than as part of a circuitboard):

Metal and semiconductor nanowires are defined as wires with diametersbelow 50 nanometers (typically 2 to 20 nanometers), and with lengths inthe range of 0.1 micrometers to 50 micrometers (typically 5 to 10micrometers). These may be prepared chemically using any one of a numberof techniques described in the references given below.

One example of a reported technique for the production of semiconductornanowires of the semiconducting element germanium is to react germaniumtetrachloride and phenyltrichlorogermanium with a dispersion of sodiummetal in the solvent toluene, and at a temperature near 300° C. in aclosed vessel, under an inert environment, for a period of several days.That preparation produces single-crystal germanium nanowires ofdiameters three to thirty nanometers, and of lengths from 0.5 to 10micrometers.

A second example of a reported technique for the production ofsemiconductor nanowires of the semiconducting element silicon, is tolaser vaporize a target containing elemental silicon and iron. Thetarget is placed in a vacuum oven at 1300° C., and an inert gas isflowed through the oven during the vaporization process. This techniqueproduces silicon wires that have diameters in the range of 20 to 30nanometers, and lengths ranging from 1 to 20 micrometers.

One example of a reported technique for the production of metallicnanowires of the metallic element gold is to electrochemically grow goldwires within the pores of an anodically etched aluminum oxide thin film.The aluminum oxide is dissolved in acidic solution, releasing the goldnanowires, which are then collected. Gold nanowires grown in this mannerare characterized by diameters ranging from 20 to 30 nanometers, andlengths ranging from 0.5 to 5 micrometers.

Nanowires of various metallic and semiconducting materials may beprepared in a variety of fashions that are listed below. Some of thesedevices will require doped semiconductor wires, such as doped Si.

For the case of Si wires, the wires can be doped when the wires arephysically prepared. In this case, it is necessary to add the dopantinto the reaction vessel as the wires are formed. For example, in thelaser ablation/vacuum oven preparation technique described above, asmall amount of dopant gas, such as phosphorus trihydride (PH₃) orarsenic trihydride (AsH₃) is added into the inert gas (argon, forexample) that flows through the vacuum oven during the laserablation/wire formation process.

Conversely, these wires can be modulation-doped by coating theirsurfaces with appropriate molecules—either electron-withdrawing groups(Lewis acids, such as boron trifluoride (BF₃)) or electron-donatinggroups (Lewisr bases, such as alkylamines) to make them p-type or n-typeconductors, respectively. See wire preparation routes listed below. FIG.1B depicts a coating 20 on wire 12 and a coating 22 on wire 14. Thecoatings 20, 22 may be modulation-doping coatings, tunneling barriers(e.g., oxides), or other nano-scale functionally suitable materials.Alternatively, the wires 12, 14 themselves may be coated with one ormore R species 16, and where the wires cross, R_(s) 18 is formed. Or yetalternatively, the wires 12, 14 may be coated with molecular species 20,22, respectively, for example, that enable one or both wires to besuspended to form colloidal suspensions, as discussed below.

To dope the wires via modulation-doping, it is necessary to chemicallyfunctionalize the surface of the wires using organic or inorganicmolecules that will covalently bind to the Si—O—H groups at the surfaceof the wires. When silicon nanowires are exposed to air, a thin surfacelayer (1 nm) of SiO₂ will naturally form, and at the SiO₂/air interface,the SiO₂ surface is terminated by Si—O—H bonds. Groups that will bind toor replace Si—O—H groups are not limited to but includeR—Si(CH₃)_(x)(OCH_(3-x)), R—Si(CH₃)_(x)(OCH₂CH_(3-x)),R—Si(CH₃)_(x)Cl_(3-x), and others. In this case, R represents an organicor inorganic moiety that can contain electron-withdrawing (a Lewis acid)or electron-donating groups (a Lewis base). This chemistry of bindingmolecules to a SiO₂ passivated silicon surface is well established. Onepublished example reaction for binding molecules to the surface of SiO₂passivated silicon is:Si—O—H_((surface))+R—Si(CH₃)₂Cl→Si—O—Si(CH₃)₂R+HCl

Other semiconductor wires can be functionalized with organo-amines,organo-thiols, organo-phosphates, etc.

No previous description of how to modulation-dope chemically synthesizedsemiconductor nanowires has yet appeared in the literature.

For the case of other nanowires, such as metal nanowires, the wires canbe chemically functionalized with R—SH (for gold or silver wires), orR—NH₂ (for platinum wires and palladium wires), or R—CO₂H for othermetals such as Al₂O₃-coated aluminum wires or titanium wires), where theR-group denotes some organic moiety that will lend the wire certainchemical properties—such as the property that will allow the personskilled in the art to disperse the wires, as a colloid, in a solvent. Inone example, gold wires might be functionalized with dodecanethiol(C₁₂H₂₅SH). The dodecanethiol not only will provide the wires with athin surface tunneling barrier, but will also allow for the wires to bedispersed in simple organic solvents, such as hexane or chloroform.

Wire Preparation Routes

The following materials may be prepared as nanowires according to thereference listed.

1. Silicon: A. M. Morales et al, “A laser ablation method for thesynthesis of crystalline semiconductor nanowires”, Science, Vol. 279,pp. 208-211 (Jan. 9, 1998).

2. Germanium: J. R. Heath et al, “A liquid solution synthesis of singlecrystal germanium quantum wires”, Chemical Physics Letters, Vol. 208,pp. 263-268 (Jun. 11, 1993).

3. Metal Nanowires: V. P. Menon et al, “Fabrication and Evaluation ofNanoelectrode Ensembles”, Analytical Chemistry, Vol. 67, pp. 1920-1928(Jul. 1, 1995).

4. Functionalizing Silicon: T. Vossmeyer et al, “Combinatorialapproaches toward patterning nanocrystals”, Journal of Applied Physics,Vol. 84, pp. 3664-3670 (Oct. 1, 1998) (one of a number of references).

5. Functionalizing the Surfaces of Gold Nanostructures: D. V. Leff etal, “Thermodynamic Size Control of Au Nanocrystals: Experiment andTheory”, The Journal of Physical Chemistry, Vol. 99, p. 7036-7041 (May4, 1995).

Molecular switching components may come from any number of differentclasses of molecules, depending, again, on the desired properties of thedevice. The key requirement of the molecules is that, when they aresandwiched between two wires, they may be electrochemically modified(i.e. oxidized or reduced) by applying a voltage across the wires. Whenthe molecular components are so modified, the net effect is that thetunneling barrier between the two wires is modified, and the rate ofcurrent flow is changed. This forms the basis of a switch that can, inturn, be used for memory, logic operations, and communication and signalrouting networks. Molecular switches can include redox pairs ofmolecules, in which application of a voltage reduces one of themolecules and oxidizes the other. An example of such a molecular redoxpair might be: nickelocene (di-cyclopentadienyl nickel), or Cp₂Ni, withtetra-butylammonium hexafluorophosphate (Bu₄NPF₆). The reaction, then,would be:(reduction) Cp₂Ni+Bu₄NPF₆→Cp₂Ni⁻+Bu₄NPF₆ ⁺  (−1.7 V)or(oxidation) Cp₂Ni+Bu₄NPF₆→Cp₂Ni⁺+Bu₄NPF₆ ⁻  (−0.1 V)

The nickelocene system is of particular interest in that the reduction,as probed by solution phase cyclic voltammetry, is highly asymmetric.Such asymmetry is analogous to magnetization hysteresis curves that formthe basis for stable and rewriteable magnetic memory. However, in thepresence of oxygen, the reduction of nickelocene is irreversible, asprobed by solution phase voltammetry. In either case, reduction oroxidation of this system will modify the tunneling barrier between thetwo wires between which the molecules are sandwiched. Thus, this systemcould operate as either a reconfigurable, or a singly configurablemolecular switch. For metallocene systems, see, e.g., J. D. L. Hollowayet al, “Electron-transfer reactions of metallocenes: Influence of metaloxidation state on structure and reactivity”, Journal of the AmericanChemical Society, Vol. 101, pp. 2038-2044 (Apr. 11, 1979).

The connector species 16 comprises a material that displays asignificant, or measurable, hysteresis in its current-voltage curve,obtained either from solution electrochemistry or from current-voltagecharacteristics in a solid-state junction. Examples of such speciesinclude metalocenes, rotaxanes, pseudo-rotaxanes, and catenanes.

The present invention can be utilized to form a useful device in any oneof three ways. First, if at least one of the wires is a dopedsemiconductor, then resonant tunneling between the two wires through theelectronic states of the molecules will form a resonant-tunneling diodethat, among other things, might serve as an inverter logic element.Second, if the redox pair can be reversibly oxidized or reduced, and ifthere is voltage hysteresis in the oxidation or reductioncurrent/voltage scan, then the device forms the basis for a randomaccess memory element, a re-settable switch, molecular logic gates, or asignal communication/routing network. Finally, if the redox pair can beirreversibly oxidized or reduced, then the device forms the basis for aread-only memory element, a singly configurable switch, logic gates, anda signal communication/routing network.

For micrometer-scale wires, devices made from redox pairs could beprepared according to the method depicted in FIGS. 2A-2D. An insulatingsubstrate 24 (SiO₂, or example) is coated with a photosensitive resist26 and then covered with a shadow mask 28 and exposed to light 30, asillustrated in FIG. 2A. The exposed pattern is developed, and a metallicwire 12 (Al, for example) is deposited onto the substrate 24. A thin (1to 2 nm) insulating layer 20 (Al₂O₃) is formed on the Al surface—in thiscase by simple exposure of the patterned substrate to air, as shown inFIG. 2B. Next, a redox pair 16, labeled R in the figure, is depositedeither by chemically selective deposition onto the Al₂O₃, as a Langmuirfilm over the entire substrate, or by sublimation of the molecules ontothe entire substrate. In the latter case, redox pairs exist both on 16 aand off 16 b the deposited wire 12 and its insulating layer 20, as shownin FIG. 2C. Next, a second wire 14 is deposited perpendicular to thefirst wire 12 through a shadow mask. The second wire 14 may include abuffer layer 38 (Ti or Cr, for example) which will form an interfacewith the deposited molecules, followed by the thicker wire 14′ depositedon top of the buffer layer, or it may just consist of a single wire 14.Only where the two wires 12, 14 cross is a device 10 defined, since anapplication of a voltage across the two wires is necessary to addressthe device. Thus, as long as the two wires 12, 14 intersect, no furtheralignment of the two lithographic steps is necessary in order to make asingle device 10.

For nanometer scale wires, devices made from redox pairs could beprepared according to the method depicted in FIGS. 3A-3C. On this case,a metal (i.e., gold) or semiconductor (i.e., silicon) nanowire 12,possibly within an insulating surface layer 20 (for silicon, this is thenaturally occurring SiO₂; for gold, this can be an alkylthiol molecularlayer) is deposited on a substrate 34, as illustrated in FIG. 3A.Second, a redox pair of molecules 16 (labeled R in FIG. 3B) istransferred as either a Langmuir-Blodgett film, or via some other formof deposition such as vacuum sublimation. The redox pair 16 can coverboth the wire 12 and the substrate 34. In the last step, either a metalor a semiconductor nanowire 14, possibly with an insulating layer (notshown), is deposited across the first wire 12. Only those redox pairs 18that are sandwiched between the two wires 12, 14 are defined, or canfunction, as molecular switches 10, as illustrated in FIG. 3C.

In one realization of the present invention, the functional groups onthe molecular wires fit together like a lock and key. One wire 12 iscoated with one molecular species 20, and the other wire 14 is coatedwith the other molecular species 22. By the simple process of crossingone type of coated wire over another, the two types of moleculesrecognize and form links R_(s) to each other, thus connecting the twowires at a point, and forming a redox pair at that point.

In another realization, one wire 12 is deposited on a substrate, a setof molecular switches is uniformly deposited over the entire substrate,and then a second wire 14 is laid across the first wire. By the simpleprocess of crossing one type of wire over another, a switch 10 isdefined at the crossing point of the two wires 12, 14, since it is onlyat that crossing point that a voltage can be applied to the molecularswitch.

The remaining molecular switch material could be washed off thesubstrate, reacted chemically, or simply left in place, depending onsubsequent processing requirements. One example of such a device 10would be to use the scheme described in FIGS. 2A-2D, in which analuminum electrode 12 is deposited using conventional lithographytechniques. Upon exposure of the electrode to air, a thin (1 nm) Al₂O₃layer 20 naturally forms on the surface of the electrode. Second, a thinfilm 16 of nickelocene is deposited by vacuum sublimation. Finally, asecond metal electrode 14 (gold, for example) is deposited perpendicularto the first electrode 12 through a lithographically defined shadowmask. A device 10 fabricated in this way might serve as a reconfigurableswitch in which the ‘switch-closed’ state was a resonant tunnelingresistor, and the ‘switch-open’ state was a tunneling resistor. Thisdevice could form the basis of a random access memory, a communicationsand signal routing network, and a configurable logic network.

In yet another realization, the junction 18 between each pair ofcrossing wires 12, 14 is a small electrochemical cell that functions asa battery. For operation, the two electrodes 12, 14 are held at aconstant potential with respect to each other because the structure ofthe battery separates two parts of an oxidation-reduction reaction by aninsulating barrier, and the only way for the chemical reaction toproceed is to allow current to flow from one electrode to anotherthrough an external circuit. The general requirement to form a batteryis the two wires or electrodes have to be composed of different elementsor compounds. In the junctions described here, each electrode material12, 14 should have an oxide coating 20, 22, respectively, and may alsohave an intervening layer of molecules 16 or material that acts toisolate the two electrodes with their oxides from each other (or thisisolating may be achieved if the two oxides are thick enough). Thebattery is charged by applying a voltage of the appropriate polarity andmagnitude to drive the junction chemical reaction in the oppositedirection that would occur naturally in the junction 18, if thematerials were mixed directly. The battery is drained either by shortingthe appropriate wires externally or by applying an external voltage toovercome kinetic barriers to the reaction to allow it to go tocompletion. The battery function of the junction is in effect in serieswith one of the other functions described for such junctions, e.g.,tunneling resistor, etc.

EXAMPLES

The present invention was reduced to practice to make a molecularswitch-based based device that could be configured as an electricallyconfigurable read-only-memory, nonlinear logic gates that operated asdiode-based logic, or a signal routing device. The device was madeaccording to the method described in FIGS. 2A-2D, with the followingmodifications: A 5 micrometer wide aluminum wire 12 was deposited on asilica substrate 24 using conventional lithographic procedures. The wire12 was provided with an oxide coating 14 of Al₂O₃, which naturallyformed to a thickness of about 1.0 to 1.5 nm when the aluminum wire wasexposed to air. One of the molecular species shown in FIGS. 4A-4C wasdissolved in tetrahydrofuran solvent, prepared as a Langmuir monolayer,and transferred as a Langmuir-Blodgett single molecular monolayer film16 that covered the Al wire 12 and the silica substrate 24. Themolecular compounds shown in FIGS. 4A-4C are from a class of molecularcompounds known as rotaxanes. Each molecular compound consists of adumbbell component 36, a counterion 38, and 0, 1, or 2(bis-para-phenylene-34-crown-10) rings 40 (FIGS. 4A, 4C, and 4B,respectively).

The conditions for the preparation of the Langmuir monolayer 16 were asurface pressure of 28 milliNewtons/meter which yields a 1 nm²/moleculesurface coverage. Previous work on LB films of similar molecules hasbeen reported; see, D. B. Amabilino et al, “Aggregation ofself-assembling branched [n]-rotaxanes”, New Journal of Chemistry, Vol.22, No. 9, pp. 959-972 (Sep. 11, 1998).

A second, top wire 14 was deposited perpendicular to the first wire 12.The second wire 14 was deposited through a shadow mask using electronbeam deposition techniques while the substrate 24 was held at roomtemperature. For this top wire 14, a thin (5 nm) titanium layer 32 wasfirst deposited across the first wire 12, sandwiching a certain part 18of the molecular monolayer 16 between the Ti layer 32 and the lower Alwire 12.

Next, the top wire of aluminum 14′ was deposited directly on top of the11 micrometer wide Ti wire 32 through the same shadow mask to anapproximate thickness of one micrometer. This titanium/aluminumelectrode 14 could be substituted with titanium/gold orchromium/aluminum electrodes, without affecting the basic deviceproperties. Over 100 devices were made this way from each of the threemolecular compounds shown in FIGS. 4A-4C, and the yield of operationalswitch junctions for each case was greater than 90%.

The electronic energy level diagram of the device 10 is shown in FIG. 5.The energy levels (Fermi level) of the bottom aluminum electrode 12 andthe top titanium/aluminum electrode 14 are indicated at either side ofthe diagram. The Al₂O₃ passivating layer 20, and the titanium/molecularinterface layer 42 are indicated as tunneling barriers. The energylevels of one of the molecular compounds (FIG. 4C) (this particularmolecule contained a dumbbell 36, a counterion 38 and a single ring 40)are indicated as either empty reducing levels 44 or as filled oxidationstates 46. These energy levels were measured for this molecule usingsolution phase pulsed voltammetry. The operation of this device isdemonstrated in FIGS. 6A-6B. For all device measurements, thetitanium/aluminum electrode 14 was held at ground.

If a negative voltage is applied across the two wires 12, 14, thencurrent tunnels between them. If the applied voltage is such that theempty electronic states 44 of the rotaxane are ‘lined up’ with the Fermilevels of one of the wires 14, then current flow is greatly enhanced bya resonant tunneling process, and a ‘high’ rate of current flow ismeasured in the area of the curve denoted 48 in FIG. 6A. This is definedas the ‘switch closed’ state. The normalized density-of-statesmeasurement of this resonant tunneling process 50 (FIG. 6B) reveals thattunneling is occurring through the empty electronic states 44 of therotaxane, thereby forming a resonant tunneling resistor that acts as aclosed switch.

If a positive voltage is applied across the two wires 12, 14, thencurrent again tunnels between them. If the applied voltage is such thatthe filled electronic states 46 (see also Curve 52 in FIG. 6B) of themolecules are ‘lined up’ with the Fermi levels of one of the electrodes14, then the rotaxane is oxidized (see the portion of the curve denoted54 in FIG. 6A). Oxidation of the rotaxane is irreversible, andphysically changes its chemical structure, and thus the molecularelectronic energy levels in the device 44, 46. When a second currentvoltage scan from 0 to −2 Volts, denoted 56 in FIG. 6A, is again appliedacross the wires 12, 14, no resonant tunneling occurs from 0 to −1 Vbecause the empty electronic states 44 of the rotaxane are no longeravailable (see Curve 58 in FIG. 6B). This forms a tunneling resistorthat acts as an open switch. This is seen in the plot of the normalizeddensity of states 58 for this scan 56. As a result, a ‘low’ rate ofcurrent flow is measured at 56.

The foregoing description is that of a singly configurable molecularswitch, and can form the basis of certain types of memory, signalrouting network, and certain types of logic circuits. The performance ofthis molecular switch is such that, at −1.8 volts, the ratio of thecurrent flow between a switch ‘closed’ state 48 and a switch ‘open’state 56 is a factor of sixty to eighty, depending on the particulardevice.

In the ‘switch-closed’ state, this device could be utilized as aresonant tunneling resistor (with an aluminum bottom electrode 12 and atitanium/aluminum top electrode 14), or a resonant tunneling diode (witha semiconductor, e.g. silicon, bottom electrode 12 and a titanium/goldtop electrode 14). In the ‘switch-open’ state, this device was atunneling resistor. A device that switches between a resonant tunnelingresistor and a tunneling resistor or between a resonant tunneling diodeand a tunneling resistor can be utilized to generate wired-logic gates,communications and signal routing circuitry, and programmable read-onlymemory. The fact that the electronic switching properties of thesemolecules is the same whether they are isolated molecules in solution(as probed by pulsed voltammetry), or as the junction molecules in thesesolid-state devices, indicates that these devices will scale from themicrometer-scale dimensions of the working devices discussed here, tonanometer, or molecular scale dimensions.

The technology disclosed and claimed herein for forming crossed wires(micrometer or nanometer) may be used to perform a variety of functionsand to form a variety of useful devices and circuits for implementingcomputing on a microscale and even on a nanoscale. Molecular wirecrossbar interconnects (MWCI) for signal routing and communications aredisclosed and claimed in related application Ser. No. 09/280,225 (U.S.Pat. No. 6,314,019); molecular wire crossbar memory is disclosed andclaimed in related application Ser. No. 09/280,189 (U.S. Pat. No.6,128,214); molecular wire crossbar logic (MWCL) employing programmablelogic arrays is disclosed and claimed in related application Ser. No.09/282,045; a demultiplexer for a MWC network is disclosed and claimedin related application Ser. No. 09/282,049 (U.S. Pat. No. 6,256,767);and molecular wire transistors are disclosed and claimed in relatedapplication Ser. No. 09/280,188, all filed on even date herewith.

As illustrated in FIG. 7, the switch 10 of the present can be replicatedin a two-dimensional array to form a plurality, or array, 60 of switchesto form a crossbar switch. FIG. 7 depicts a 6×6 array 60, but theinvention is not so limited to the particular number of elements, orswitches, in the array. Access to a single point, e.g., 2 b, is done byimpressing voltage on wires 2 and b to cause a change in the state ofthe molecular species 18 at the junction thereof, as described above.Details of the operation of the crossbar switch array 60 are furtherdiscussed in related application Ser. No. 09/280,225 (U.S. Pat. No.6,314,019).

Thus, there has been disclosed chemically synthesized and assembledelectronic devices comprising crossed wires joined by electrochemicallyswitchable molecular species at the intersecting junctions. It will beapparent to those skilled in this art that various changes andmodifications of an obvious nature may be made, and all such changes andmodifications are considered to fall within the scope of the appendedclaims.

1. A method of fabricating a crossed-wire device comprising a pair ofcrossed wires which form a junction where one wire crosses another andat least one connector species connecting said pair of crossed wires insaid junction, said junction having a functional dimension innanometers, wherein said at least one connector species and said pair ofcrossed wires forms an electrochemical cell, said method comprising (a)forming said first wire, (b) depositing said at least one connectorspecies over at least a portion of said first wire, and (c) forming saidsecond wire over said first wire so as to form said junction, whereinsaid at least one connector species comprises an electricallyaddressable molecular species.
 2. The method of claim 1 wherein said atleast one connector species forms a quantum state molecular switchcomprising an electrically adjustable tunnel junction between said twowires.
 3. The method of claim 1 wherein at least one of said two wiresis formed to a thickness that is about the same size as said at leastone connector species, and over an order of magnitude longer than itsdiameter.
 4. The method of claim 3 wherein both of said two wires areformed to a thickness that is about the same size as said at least oneconnector species.
 5. The method of claim 1 wherein both of said twowires are formed to a thickness that ranges from sub-micrometer tomicrometer.
 6. The method of claim 1 wherein said junction forms asingly configurable or reconfigurable switch.
 7. The method of claim 6wherein said junction is at least one of elements selected from thegroup consisting of resistors, tunneling resistors, diodes, tunnelingdiodes, resonant tunneling diodes, and batteries.
 8. The method of claim1 wherein each said wire independently comprises a conductor or asemiconductor.
 9. The method of claim 8 further including forming aninsulating layer or a modulation-doped coating on at least one of saidwires.
 10. The method of claim 1 wherein said at least one connectorspecies comprises a bi-stable molecule.
 11. The method of claim 10wherein said bi-stable molecule is one that displays a significanthysteresis in its current-voltage curve, obtained either from solutionelectrochemistry or from current-voltage characteristics in asolid-state junction.
 12. The method of claim 10 wherein said device isirreversibly switchable from a first chemical state to a second chemicalstate of said bi-stable molecule.
 13. The method of claim 10 whereinsaid device is reversibly switchable between a first chemical state anda second chemical state of said bi-stable molecule.
 14. The method ofclaim 10 wherein said connector species comprises a layer of saidbi-stable molecules.
 15. The method of claim 14 wherein said layer ofsaid bi-stable molecules has a thickness of a monolayer of saidbi-stable molecules.
 16. The method of claim 10 wherein said connectorspecies is selected from the group consisting of metalocenes, rotaxanes,pseudo-rotaxanes, and catenanes.